Printed circuit boards (PCBs), also called printed wiring boards (PWBs) are used to electrically connect one or more electronic components. Electrical devices are usually attached to the top or bottom of the printed circuit board. Conductive material called a “trace” connects the electrical devices mounted to the printed circuit board. Conductive planes may be placed above or below the traces. The traces and the planes may be separated by a dielectric or non-conductive material. The conductive planes may be used for a plurality of different purposes. For example, the conductive planes may be used as an electrical ground plane functioning as an electrical ground return path. The conductive plane may also be used as an electrical power plane functioning as a power distribution path. Together, the trace and plane provide a complete electrical path to and from electrical components associated with a printed circuit board. Generally speaking, traces typically connect signal paths of electronic components associated with a printed circuit board, while planes typically connect power or ground paths of electrical components associated with a printed circuit board. A cross-section of a simple example of a printed circuit board is depicted in FIG. 1.
Trace layers, dielectric or non-conductive layers, and plane layers may be repetitiously laminated to increase the number of traces available for connecting electrical components. A typical cross section of printed circuit board is shown below in FIG. 2.
Minimizing layers is desirable to reduce the cost of a printed circuit board. A power plane or ground plane may often be segmented or discontinuous. One segment of a plane may be used to distribute a voltage, while another segment is used to distribute a different voltage or to serve as an electrical ground. These segments may be coplanar. A segmented ground plane may provide a current path or current return path for more than one voltage. A typical cross section of printed circuit board with split planes is shown below in FIG. 3.
An electrical driver may electrically raise or lower a voltage value of an associated trace. The voltage value often corresponds to a logic value. As the electrical driver attempts to change the voltage value of the trace, a charging current propagates down the trace. A wave edge is created. The edge represents a change in current and a change in voltage. These changes are accompanied by electric fields. As the electric signal propagates down the trace, power or ground planes may provide a return path for current and electric fields. Electric fields produced by the propagating signal follow the signal down the trace. These electric fields may couple to nearby traces and induce an undesired artifact on other signals. See FIG. 4
The induced signals travel in approximately the same direction as the original signal when traces are approximately parallel. Proper design dictates spacing traces far enough apart to reduce or negate the undesired coupling when possible. See FIG. 5.
When power planes or ground planes are discontinuous (split, segmented), propagating signals that cross the split encounter a discontinuity in the conductive plane return path. The discontinuity effects the signal and the associated electric fields. As the leading edge of the signal approaches the area of a plane split, the associated electric fields spread away from the originating trace, and may induce an undesired signal on a nearby trace (See FIG. 6). Additional separation of the traces is usually impractical to avoid this increased coupling, since the area of the printed circuit board is limited and costly.
Electronic circuits receiving signals of effected traces may sample erroneous electrical values at their inputs. Sampling an erroneous value is undesirable. Another effect of the discontinuous plane is a change in the electrical impedance of the trace. Since geometry is a factor in the impedance of a trace, a change in geometry causes a change in the trace impedance. A change of impedance causes reflections of the propagating signal.
Therefore, a need existed to provide an improved printed circuit board and method to overcome the above problem. The improved printed circuit board and method must minimize cross talk between signals on the printed circuit board. The improved printed circuit board and method must further minimize reflections due to split planes on the printed circuit board.